Load snap-in Active Directory Schema
It will then connect to the old AD server first
Right click ‘Connect to Schema Operations Master’ and select the new server
Then select Operations Master and change the role with the new server:

Demoting old server

Obviously, go to the old server;
Use dcpromo.exe, it will say AD is a Global Catalog server;
Go to Active Directory Users and Computers (this can be done from either new server or old server);
Expand the domain and select Domain Controllers;
Select the old server and click on NTDS Settings;
Untick Global Catalog;

And now we should be able to demote the old server.

If you have any questions or comments, please write down below and I would happy to answer 🙂